Method of manufacturing polystyrene foam with polymer processing additives

ABSTRACT

Disclosed is a method for making polystyrene foam which utilizes one or more atmospheric gases, particularly CO 2 , as the blowing agent in combination with a polymer processing aid (PPA), typically an ester that is relatively non-volatile at the extrusion temperature range. The blowing agent and the PPA may both be introduced into the molten thermoplastic polystyrene resin or the PPA may be incorporated in the solid source polystyrene resins. The resulting foam will be substantially free of residual blowing agent and dimensionally stable at ambient temperatures.

RELATED APPLICATIONS

This application is a divisional application of U.S. Ser. No.11/259,970, filed Oct. 27, 2005 titled METHOD OF MANUFACTURINGPOLYSTYRENE FOAM WITH POLYMER PROCESSING ADDITIVES, the entiredisclosure of which is incorporated herein by reference.

REFERENCE TO GOVERNMENT RIGHTS

This invention was made with Government support under AdvancedTechnology PROGRAM (ATP) Grant No. 70NANB2H3023 awarded by the NationalInstitute of Standards and Technology (NIST). The Government may havecertain rights to this invention.

TECHNICAL FIELD AND INDUSTRIAL APPLICABILITY

This invention relates to processes for forming polymeric foams,particularly to the manufacture of extruded polystyrene (XPS) foams inthe absence of chlorofluorocarbon and fluorocarbon blowing agents byusing one or more esters, particularly adipates, benzoates anddibenzoates, as polymer processing aids (PPA) for improving theappearance and properties of the resulting foam, and more particularlyto processes for preparing extruded polystyrene foam products frompolystyrene blends using carbon dioxide as a primary blowing agent.

The invention relates to compositions and methods for producing extrudedpolystyrene (XPS) foam board suitable for insulation applications,particularly for exterior insulation finish system (EIFS) for buildingconstruction.

BACKGROUND OF THE INVENTION

In the traditional production of polystyrene (PS) foams using anextrusion process, it was common to utilize as blowing agents one ormore halocarbons, such as methyl chloride, ethyl chloride,chlorocarbons, fluorocarbons (including HFCs) and chlorofluorocarbons(CFCs) including dichlorodifluoromethane, fluorohydrocarbons orchlorofluorohydrocarbons (which are also referred to as “soft CFCs”,“HCFCs” or “HFCs”). Examples of such halocarbons include a range of CFCssuch as CFC-11 (chlorotrifluoromethane), CFC-12(dichlorodifluoromethane), and CFC-113(1,2,2-trifluoro-1,1,2-tri-chloroethane), and hydrohalocarbons, alsoreferred to as “soft” CFCs, HCFCs and HFCs, including HCFC-22(chlorodifluoromethane), HCFC-123 (1,1-dichloro-2,2,2-trifluoroethane),HCFC-142b (1-chloro-1,1-difluoroethane), HFC-134a(1,1,1,2-tetrafluoroethane), HFC-152a (1,1-difluoroethane), andHCFC-141b (1,1-dichloro-1-fluoroethane.

The general procedure utilized in the preparation of extruded syntheticfoam bodies generally includes the steps of melting a base polymericcomposition, incorporating one or more blowing agents and otheradditives into the polymeric melt under conditions that provide for thethorough mixing of the blowing agent and the polymer while preventingthe mixture from foaming prematurely, e.g., under pressure. This mixtureis then typically extruded through a single or multi-stage extrusion dieto cool and reduce the pressure on the mixture, allowing the mixture tofoam and produce a foamed product. As will be appreciated, the relativequantities of the polymer(s), blowing agent(s) and additives, thetemperature and the manner in which the pressure is reduced will tend toaffect the qualities and properties of the resulting foam product.

The solubility of chlorofluorocarbons and certain alkanes in polystyrenetends to reduce the extrusion melt viscosity and improve cooling ofexpanded polystyrene (PS) melts. For example, the combination of pentaneand a chlorofluorocarbon such as Freon 11 and 12 is partially soluble inpolystyrene and has been used for generating polystyrene foams thatexhibited a generally acceptable appearance and physical properties suchas surface finish, cell size and distribution, orientation, shrinkageand stiffness.

However, in response to the apparent contribution of such CFC compoundsto the reduction of the ozone layer in Earth's stratosphere, thewidespread use and accompanying atmospheric release of such compounds inapplications such as aerosol propellants, refrigerants, foam-blowingagents and specialty solvents has recently been drastically reduced oreliminated by government regulation. Although certain of the “soft” CFCssuch as certain hydrofluorocarbons (HFC's) including1,1,1,2-tetrafluoroethane (HFC-134a) and 1,1-difluoroethane (HFC-152a)are thought to be much more ozone friendly and have been considered asalternative blowing agents. However, these alternative compounds areexpensive, tend to be less soluble in polystyrene, tend to have higherthermal conductivity than HCFC's and may still contribute to globalwarming.

Hydrocarbons such as pentane, hexane, cyclopentane and other homologs ofthis series have also been considered, but they are highly flammable andvolatile, thereby raising both safety and VOC emission concerns. Carbondioxide is an attractive candidate as a blowing agent, from both theenvironmental and economic standpoints. The challenges associated withsuccessfully using CO₂ as a blowing agent are, however, significant inlight of the relatively low solubility, high diffusivity and poorprocessability of CO₂ in polystyrene resins. CO₂ also has an increasedthermal conductivity relative to that of HCFC-142b and HFC-134a, withCO₂-blown foam exhibiting about 17% and about 10% lower overall productinsulation values respectively than corresponding foams produced withHCFC-142b and HFC-134a.

Other previous attempts have utilized alcohols, such as ethanol, orhydrocarbon, such as cyclopentane, in conjunction with CO₂ to improvethe processability and enable the production of extruded polystyrenefoam board having desired or target physical and thermal properties. Theproblems with co-blowing agents such as alcohols or hydrocarbons aretheir flammability, safety and the negative impact on flame performanceand insulation properties of the end product.

Conventional processes include polymer foam processes for makingthermoformed articles wherein the blowing agent comprises a mixture ofat least an atmospheric gas and at least one volatile plasticizingblowing agent. Previous attempts to mix normally liquid hydrocarbons andnormally gaseous blowing agents have generally achieved only limitedsuccess and have tended to require great care in order to produceacceptable polymer foams using highly-volatile blowing agents such ascarbon dioxide.

SUMMARY OF THE INVENTION

The objectives of the present invention include providing an improvedmethod of making polymeric foams using one or more atmospheric gases asa blowing agent in combination with at least one ester, particularly anadipate ester, as a polymer processing aid.

In an exemplary embodiment of the invention, extruded polystyrene foamsare prepared from a polymeric melt, typically one that includes a highmelt flow polystyrene as the primary polymeric component, an atmosphericgas such as CO₂ as the primary blowing agent and at least one polymerprocessing aid selected from a group of esters, especially adipateshaving a bis(n-R) structure, wherein R is aliphatic (linear, cyclic andbranched, saturated and unsaturated) or aromatic with a preferredadipate being bis(n-decanyl) adipate.

In an exemplary embodiment of the invention, the polymer melt will beprepared from a major portion of one or more styrenic polymers thatexhibit a high melt index, e.g., a melt index of at least about 10.0(g/10 minutes) (as measured according to ASTM D 1238, Condition L) thatis combined with no more than about 5 wt % of bis(n-decanyl) adipate asa polymer processing aid and less than about 4 wt % CO₂ as a blowingagent,

In an exemplary embodiment of the invention, the polymer melt will beprepared from preformed styrenic polymers or copolymers that may havebeen precompounded with bis(n-decanyl) adipate. Alternatively oradditionally, the bis(n-decanyl) adipate can be injected directly intothe polymeric melt at an intermediate position along the screw extruderpath. In addition to the bis(n-decanyl) adipate, other processing aidsmay be incorporated in the preformed styrenic polymers or may beinjected into the melt as it moves through the extruder. The blowingagent(s), such as CO₂, is also injected into the polymeric melt at anintermediate portion along the screw extruder path. In any event, eachof the additives and blowing agent(s) should be introduced into thepolymeric melt sufficiently upstream of the extrusion die to ensure thatadequate blending is achieved before the composition reaches theextrusion die.

In accordance with the invention, the method of making polystyrene foamcomprises mixing an atmospheric gas such as carbon dioxide, nitrogen orair and at least one a non-volatile blending agent into a polystyrenemelt. The polystyrene melt may also include one or more nucleatingagents such as talc, sodium bicarbonate or citric acid. The mixture ofthe polystyrene melt, the atmospheric gas and the blending agent arethen emitted through an extrusion die, thereby reducing the pressure andallowing the blowing agent to expand and form polystyrene foam.Depending on the concentration of the blowing agent and the extrusionconditions, the resultant foam may have substantially no residualblowing agent and will exhibit acceptable foam and surface properties.

DESCRIPTION OF THE DRAWINGS

Example embodiments of the invention will be apparent from the moreparticular description of certain example embodiments of the inventionprovided below and as illustrated in the accompanying drawings.

FIG. 1 is a schematic drawing of an exemplary extrusion apparatus usefulfor practicing methods according to the invention;

FIG. 2 is a schematic drawing of another exemplary extrusion apparatususeful for practicing methods according to the invention; and

FIG. 3 is a chart illustrating pressure measurements obtained using avariety of comparative and demonstrative compositions and methods.

These drawings have been provided to assist in the understanding of theexample embodiments of the invention as described in more detail belowand should not be construed as unduly limiting the invention. Inparticular, the number, relative spacing, positioning, sizing anddimensions of the various elements illustrated in the drawings are notdrawn to scale and may have been exaggerated, reduced or otherwisemodified for the purpose of improved clarity.

DETAILED DESCRIPTION OF EXEMPLARY EMBODIMENTS

As illustrated in FIG. 1, an extrusion apparatus 100 useful forpracticing methods according to the invention may comprise a single ordouble (not shown) screw extruder including a barrel 102 surrounding ascrew 104 on which are provided a spiral flight 106 configured tocompress, and thereby, heat material introduced into the screw extruder.As illustrated in FIG. 1, the basic polymeric composition can be feedinto the screw extruder as a flowable solid, such as beads, granules orpellets, or as a liquid or semiliquid melt, from one or more (not shown)feed hoppers 108.

As the basic polymeric composition advances through the screw extruder,the decreasing spacing of the flight 106, define a successively smallerspace through which the polymer composition is forced by the rotation ofthe screw. This decreasing volume acts to increase the temperature ofthe polymer composition to obtain a polymeric melt (if solid startingmaterial was used) and/or to increase the temperature of the polymericmelt.

As the polymer composition advances through the screw extruder 100, oneor more ports may be provided through the barrel 102 with associatedapparatus 110 configured for injecting one or more polymer processingaids into the polymer composition. Similarly, one or more ports may beprovided through the barrel 102 with associated apparatus 112 forinjecting one or more blowing agents into the polymer composition. Oncethe polymer processing aid(s) and blowing agent(s) have been introducedinto the polymer composition, the resulting mixture is subjected to someadditional blending sufficient to distribute each of the additivesgenerally uniformly throughout the polymer composition to obtain anextrusion composition.

This extrusion composition is then forced through an extrusion die 114and exits the die into a region of reduced pressure (which may be belowatmospheric pressure), thereby allowing the blowing agent to expand andproduce a polymeric foam layer or slab. The polymeric foam may besubjected to additional processing such as calendaring, water immersion,cooling sprays or other operations to control the thickness and otherproperties of the resulting polymeric foam product.

As illustrated in FIG. 2, an extrusion apparatus 200 useful forpracticing methods according to the invention may comprise a single ordouble (not shown) screw extruder including a barrel 202 surrounding ascrew 204 on which are provided a spiral flight 206 configured tocompress, and thereby, heat material introduced into the screw extruder.As illustrated in FIG. 2, the basic polymeric composition, optionallycompounded with one or more polymer processing aids, can be feed intothe screw extruder as a flowable solid, such as beads, granules orpellets, or as a liquid or semiliquid melt, from one or more (not shown)feed hoppers 208.

As the basic polymeric composition advances through the screw extruder,the decreasing spacing of the flight 206, define a successively smallerspace through which the polymer composition is forced by the rotation ofthe screw. This decreasing volume acts to increase the temperature ofthe polymer composition to obtain a polymeric melt (if solid startingmaterial was used) and/or to increase the temperature of the polymericmelt.

As the polymer composition advances through the screw extruder 200, oneor more ports may be provided through the barrel 202 with associatedapparatus 212 configured for injecting one or more blowing agents and,optionally one or more polymer processing aids, into the polymercomposition. Once the desired quantities of polymer, polymer processingaid(s) and blowing agent(s) have been introduced into the screwextruder, the resulting mixture is subjected to some additional blendingsufficient to distribute each of the additives generally uniformlythroughout the polymer composition to obtain an extrusion composition.

This extrusion composition is then forced through an extrusion die 214and exits the die into a region of reduced pressure (which may be belowatmospheric pressure), thereby allowing the blowing agent to expand andproduce a polymeric foam layer or slab. As illustrated in FIG. 2, thispressure reduction may be obtained gradually as the extruded polymericmixture advances through successively larger openings provided in thedie or through some suitable apparatus (not shown) provided downstreamof the extrusion die for controlling to some degree the manner in whichthe pressure applied to the polymeric mixture is reduced. The polymericfoam may also be subjected to additional processing such as calendaring,water immersion, cooling sprays or other operations to control thethickness and other properties of the resulting polymeric foam product.

Exemplary methods according to the invention may utilize one or more ofa variety of blowing agents to achieve the desired polymeric foamproperties in the final product. In general, the polymeric compositionwill include at least a major portion of a high melt flow polystyrene(e.g., a polystyrene having a melt flow index of at least about 10 g/10minutes (as measured according to ASTM D 1238 Condition L) using anatmospheric gas, preferably CO₂, as the primary blowing agent.

In addition to the CO₂, one or more polymer processing aids (PPA)selected from a group of esters, particularly adipate esters, and moreparticularly bis(n-R) adipate esters, wherein R is selected from a groupconsisting of C₆-C₁₆, and preferably C₈-C₁₃, aliphatic (linear, cyclicand branched, saturated and unsaturated) and aromatic (substituted andunsubstituted) groups, particularly compounds such as bis(n-decanyl)adipate. The processing aid(s) will improve the stability of theextrusion pressure/temperature profile and thereby improve theuniformity in the production of different thicknesses of polystyrenefoam board using an atmospheric gas such as air, N₂ or, preferably, CO₂as the primary blowing agent.

The polymeric composition will preferably be a styrenic polymer and/oranother polymer having a sufficiently high melt flow index (MFI or meltflow number), e.g., a melt flow index of at least about 10 (g/10minutes), thereby increasing the CO₂ solubility relative to that whichcan be achieved with polystyrenes having a MFI of less than 10. Thispolymeric composition may then be combined with minor amount of apolymer processing aid, typically an ester and preferably an adipateester according to the general Formula I provided below:

wherein X is nitrogen or oxygen, R¹ is selected from a group consistingof C₁-C₂₀ alkyl, aryl or alkaryl and R² and R³ are independentlyselected from a group consisting of hydrogen, C₁-C₂₀, preferably C₆-C₁₆,and more preferably C₈-C₁₃, aliphatic (linear, cyclic and branched,saturated and unsaturated) and aromatic (substituted and unsubstituted)groups (and are generally identical), alkaryl and alkoxylate, wherein R²and R³ cannot both be H. Representative compounds are represented byFormulas II-IV as provided below:

ADIPATE ESTER FORMULA

II

III

IV

In addition to the adipate esters detailed above, benzoates generallycorresponding to the general Formula V and reproduced below may beuseful for increasing the solubility of CO₂ in polymeric compositions,such as polystyrene.

wherein X is nitrogen or oxygen, R³ is selected from a group consistingof C₁-C₂₀ alkyl, aryl, alkaryl and alkoxylate. Suitable compoundscorresponding to the general structure illustrated in Formula V for usein this invention are illustrated below as Formulas VI-X:

In addition to the adipate esters and benzoates detailed above,dibenzoates generally corresponding to the general Formula XI andreproduced below may be useful for increasing the solubility of CO₂ inpolymeric compositions, such as polystyrene.

wherein X is nitrogen or oxygen, R⁴ is selected from a group consistingof C₁-C₂₀ alkyl, aryl, alkaryl and alkoxylate. Suitable compoundscorresponding to the general structure illustrated in Formula V for usein this invention are illustrated below as Formulas XII-XVI:

A minor portion, typically less than about 5 wt %, preferably less thanabout 3 wt % or, perhaps, even less than about 2 wt % of a PPA, such asan adipate polymer processing aid, can be used in combination with asimilar or greater concentration of the blowing agent(s). For example,bis(n-decanyl) adipate (Formula III) can be incorporated into apolymeric system or melt at a rates as low as about 0.5 wt % and stillexhibit improvements to the CO₂ solubility, extrusion process stabilityas reflected by temperature/pressure profiles of the process to producefoam board exhibiting improved dimensional stability. The esters, andparticularly adipate esters, will tend to outperform smaller alcoholcompounds, such as ethanol, for maintaining the properties of theresulting foam board products, particularly when CO₂ is used as the onlyblowing agent.

In those instances in which the PPA is available as a liquid at or nearroom temperature, such as bis(n-decanyl) adipate, the PPA may be pumpedthrough an injector and into an intermediate point in the movement ofthe polymeric composition through the extrusion device or extruder.Accordingly, for systems or apparatus that incorporated suitable liquidhandling equipment, such as the assemblies that were utilized to injectone or more conventional PPA's such as ethanol, these same assembliescan be utilized to inject one or more of the new PPAs.

Other PPA additives such as bis(3-ethylhexyl) adipate (Formula II) andbis(n-tridecanyl) (Formula IV) and other similar compounds tend toexhibit similar effects with regard to improving the processability ofCO₂ in the polymeric melt. Other polymers and copolymers such as styrenemethyl methacrylate (SMMA) copolymers can also be utilized as thepolymeric composition in methods according to the present invention andcan be processed on the same apparatus as general purpose crystallinepolystyrene.

With respect to copolymers in particular, utilizing bis(n-decanyl)adipate as a PAA in a SMMA copolymer composition having a S:MMA molarratio of about 80:20 is able to support about 5 wt % CO₂ in theextrusion composition. This polymeric composition and PPA additivescheme tends to increase blowing power and reduce the cooling demandsassociated with the resulting polystyrene foam board products.

Several of the PAAs, including the bis(n-decanyl) adipate, can becompounded with other polymers and copolymers such as ethylene methylacrylate and added directly to the flowable particles, beads, pellets orother compounded forms and tend to exhibit similar effects on thefoaming process in general and improving CO₂ solubility within thepolymeric composition. The PAA compound(s) can be incorporated into themelt through direct injection into the extruder or throughprecompounding (blending) the PAA compound(s) with one or more of theother compatible polymers or additives to achieve similar effectiveconcentrations in the final extrusion composition and thereby producesimilar effects.

It was also noted by the inventors that the presence of bis(n-decanyl)adipate (Formula III) appears to increase the solubility of HFC-134a inthe polymeric composition. Accordingly, presence of bis(n-decanyl)adipate helps support polymeric compositions using combinations ofblowing agents such as HFC-134a/CO₂ as well as the addition of water asa co-blowing agent to produce polystyrene foam board with desiredproperties.

Examples

A series of experiments were conducted in order to investigate therelative performance of the invention and conventional practices in theproduction of XPS products. Each of the trials used amorphous,general-purpose crystal polystyrene (specifically PS NC0038 from NOVAChemical) having a melt flow index of 5-30 (g/10 min) (ASTM D 1238Condition L), 0.78 wt % of bis(n-decanyl) adipate, 1.0 wt % ofhexabromocyclodecane as a flame retardant and 0.2 wt % talc asnucleating agent. This mixture was then fed into a twin screw extruderhaving a screw diameter of 132 mm. The solids are melted, and then mixedwith 3.7 wt % of CO₂.

From the extruder, the plastified foamable mixture was cooled to asecond temperature (generally referred to as die melt temperature) andextruded into a region of reduced pressure to form a foam product. Inthis instance, the mixture was cooled to a die melt temperature of 116°F. (about 47° C.) and was then ejected through a die opening (27 cm×1.41mm) into a region of lower pressure. Exemplary samples according to theinvention were generated by foaming the mixture under both atmosphericpressure (101.3 kPa) and subatmospheric pressure conditions,specifically a vacuum of 8, 12 or 16 inches of Hg (about 74.3, 60.8 and47.3 kPa respectively).

Comparative samples were prepared using HCFC-142b (11 wt % based on thepolymer content) as the blowing agent and using a combination of CO₂(3.7 wt %) and ethanol (1.5 wt %) as a blowing agent system. Additionalexemplary samples were produced using CO₂ (3.7 wt %) and bis(n-decanyl)adipate (Formula III) (0.5 wt %) as a blowing agent system. The lowlevel of bis(n-decanyl) adipate helps foam surface characteristics andat the same time enhances polymer melt processability The die pressurefluctuation usually is an indication of how any of the polymeric blowingagent system processes on the pilot line. As one can see from FIG. 1 theoverall pressure changes is not significant, as comparing the abovementioned systems.

FIG. 3 shows a comparison between current HCFC-142b 11%, CO₂3.7%/ethanol 1.5% and CO₂ 3.7%/bis(n-decanyl) adipate at 0.5%, 1.0% and1.5% levels with the respective extrusion out pressure, static coolerpressure and die pressure monitored on pilot line runs with thesecompositions. For the 142b system (control) general purposes PS(NOVA1220) with melt flow index of 1.6 (g/10.0 minutes) was used. ForCO₂/ethanol and all other runs PS high melt flow index 10.0 (g/10minutes) (NOVA NN0038) was been used. Attempts were made to produce foamboard have a thickness of around 1 inch (about 25.4 mm) while keepingall other process parameters relatively constant.

Although FIG. 3 shows the extrusion pressures at atmospheric condition,this relationship appears to hold true for 10 inches of Hg (about 67.5kPa) (or any other vacuum levels) as well too. These results wereachieved on an Owens Corning pilot line facility at Tallmadge, Ohio. Theextrusion pressure profile is an indication of ease of process abilityand blowing agent solubility in the melt. Although. the extrusion out,cooler and die pressures for CO₂/bis(n-decanyl) adipate system werehigher relative to the conventional 142 b system, they were verycomparable to the CO₂/ethanol system. In fact CO₂/bis(n-decanyl) adipateat 1.5% level outperforms CO₂/Ethanol system in terms of ease of processability.

Later during the trials the polymer was switched to a S:MMA 80:20 (NOVANC0044) copolymer. Because the process appeared relatively stable, weincreased the CO₂ incrementally to 4.3% to increase both the blowingpotential and the cooling capabilities. This higher CO₂ level providedacceptable products with a good surface under atmospheric conditions.Apparently the presence of the PPA bis(n-decanyl) adipate combined withmore polar and high melt index S:MMA copolymer helps increase thesolubility of CO₂ in the polymer system. The ability to dissolve moreCO₂ has huge impact on cooling power of the BA system and ease ofprocess ability. The results of the trials are reported below inTABLE 1. Average cell size, compressive modulus, percentage of opencells and R-value were all measured.

R-value, or total thermal resistance, is the measure of the resistanceof heat transfer. The method of determining R-value is described asfollows. Thermal conductivity, k is defined as the ratio of the heatflow per unit cross-sectional to the temperature drop per unit thicknesswith the US unit:

$\frac{{Btu} \cdot {in}}{{Hr} \cdot {Ft}^{2} \cdot {\,^{\circ}F}}$

And the metric unit:

$\frac{W}{m \cdot k}$

The heat transfer through an insulating material can occur through solidconductivity, gas conductivity, radiation, and convection. The totalthermal resistance (R-value), R is the measure of the resistance to heattransfer, and is determined as:

R=t/k

Where, t=thickness.

There were some notable physical properties differences between boardsmade using HCFC-142b, CO₂/ethanol and CO₂/bis(n-decanyl) adipate,Table 1. The board becomes weaker as the level of bis(n-decanyl) adipateincreases from 0.5 to 1.5%, a results which suggests a higher degree ofplasticization. Also, apparently as a result of the increasingconcentrations of bis(n-decanyl) adipate, the open cell content tendedto increase accordingly.

Additional trials were run using PS NC0038 (NOVA Chemicals) with bothHFC-134a 7%/CO₂ 0.5%/bis(n-decanyl) adipate 1.5% andHFC-134a/bis(n-decanyl) adipate 1.5% as the blowing compositions on thesame pilot line used to generate the HFC-142b discussed above. Thesetrials produced PS foams having 2.25 pcf densities with an open cellcontent of only about 2.29%, a value which is well within the acceptablerange. In both of these runs, an additional 0.2% ethyl methyl acrylatepolymer was used to improve foam board surface quality.

TABLE 1 Foam Board Properties CO₂ (3.7 wt %) HCFC- (Formula (Formula(Formula 142b Ethanol I) II) III) Average cell 0.213 0.151 0.154 0.160.172 (mm) Compressive 1261 1300 971 723 modulus (psi) Open cell, % 0.422.74 3.55 4.52 6.81 R-value 5.0 4.2 4.2 4.2 4.2 (per inch)

As noted above, the disclosed apparatus and methods of makingpolystyrene based foam products using one or more atmospheric gasesand/or a halohydrocarbon as the primary blowing agent(s) in combinationwith a polymer processing aid comprising one or more esters,particularly adipates, benzoates and dibenzoates. Further, as thepolystyrene foam is extruded, the melt can be foamed and cooled to adegree sufficient to maintain generally normal process levels and can,accordingly, be controlled with conventional practices for obtainingfoam products adapted for particular final uses. These conventionalpractices may be adapted for use with the present invention to providesome degree of control over the foam density and cell size whileutilizing conventional extrusion apparatus and post-extrusionprocessing.

Although the invention has been described in the context of particularpolystyrene foam materials, the inventive method is also applicable toother polymeric compositions and various combinations of blending agentsto obtain a variety of polymeric foam materials. Example embodiments ofthe invention have been disclosed herein and, although specific termsare employed, they are used and are to be interpreted in a generic anddescriptive sense only and not for purpose of limitation. Accordingly,it will be understood by those of ordinary skill in the art that variouschanges in form and details of the disclosed apparatus and methods maybe made without departing from the spirit and scope of the invention asset forth in the following claims.

What is claimed is:
 1. A method of manufacturing polymeric foam,comprising: preparing a polymeric melt from a major amount of a polymercomposition having a melt flow index of at least about 2 and a minoramount of at least one additive according the formula:

wherein R¹ and R² are independently selected from at least one ofhydrogen, C₁-C₂₀ aliphatic, and C₆-C₁₆ aromatic groups; injecting CO₂and one or more blowing agents into the polymeric melt to form afoamable mixture; and extruding the foamable mixture through a die intoa region of reduced pressure.
 2. A method of manufacturing polymericfoam according to claim 1, wherein the at least one additive has theformula:


3. A method of manufacturing polymeric foam according to claim 1,wherein the one or more blowing agents comprises a hydrofluorocarbon(HFC).
 4. A method of manufacturing polymeric foam according to claim 3,wherein the one or more blowing agents comprise1,1,1,2-tetrafluoroethane (HFC-134a) and 1,1-difluoroethane (HFC-152a).5. A method of manufacturing polymeric foam according to claim 1,wherein the carbon dioxide comprises less than about 4% by weight of thefoamable mixture.
 6. A method of manufacturing polymeric foam accordingto claim 5, wherein the carbon dioxide comprises between about 0.5% byweight and about 1.5% by weight of the foamable mixture.
 7. A method ofmanufacturing polymeric foam according to claim 1, wherein the at leastone additive comprises less than about 1% by weight of the foamablemixture.
 8. A method of manufacturing polymeric foam according to claim1, wherein the at least one additive comprises from about 0.5% by weightto about 1.5% by weight of the foamable mixture.
 9. A foam boardmanufactured by the method of claim
 1. 10. A method of manufacturingpolymeric foam according to claim 1, wherein the at least one additiveprovides a density of about 2.25 pcf or less in the polymeric foam. 11.A method of manufacturing polymeric foam according to claim 1, whereinthe at least one additive provides an R-value of at least 4.2°F.·ft2·hr/BTU per inch in the polymeric foam.
 12. A method ofmanufacturing polymeric foam according to claim 1, wherein the polymercomposition and the at least one additive are precompounded prior to theinjection of the one or more blowing agents.
 13. A method ofmanufacturing a polymeric foam product, the method comprising: preparinga polymeric melt from a major amount of a polystyrene polymer and aminor amount of at least one additive according the formula:

wherein R¹ and R² are independently selected from at least one ofhydrogen, C₁-C₂₀ aliphatic, and C₆-C₁₆ aromatic groups; injecting carbondioxide into the polymeric melt to form a foamable mixture; andextruding the foamable mixture through a die into a region of reducedpressure.
 14. A method of manufacturing a polymeric foam productaccording to claim 13, wherein the at least one additive has theformula:


15. A method of manufacturing a polymeric foam product according toclaim 13, wherein the at least one additive provides an R-value of atleast 4.2° F.·ft2·hr/BTU per inch in the polymeric foam product.
 16. Amethod of manufacturing a polymeric foam product according to claim 13,wherein the at least one additive provides a density of about 2.25 pcfor less in the polymeric foam product.
 17. A method of manufacturing apolymeric foam product according to claim 13, wherein the at least oneadditive provides an open cell content of about 5% or less in thepolymeric foam product.
 18. A method of manufacturing a polymeric foamproduct according to claim 13, wherein the polymer composition and theat least one additive are precompounded prior to the injection of carbondioxide.
 19. A method of manufacturing expanded polymeric foam,comprising: preparing a polymeric melt from a major amount of a polymercomposition having a melt flow index of at least about 2 and a minoramount of at least one additive according the formula:

injecting two or more blowing agents and carbon dioxide into thepolymeric melt to form a foamable mixture; and extruding the foamablemixture through a die into a region of reduced pressure
 20. A method ofmanufacturing expanded polymeric foam according to claim 19, wherein thetwo or more blowing agents comprise hydrofluorocarbons (HFC).
 21. Amethod of manufacturing expanded polymeric foam according to claim 20,wherein the two or more blowing agents comprise1,1,1,2-tetrafluoroethane (HFC-134a) and 1,1-difluoroethane (HFC-152a).22. A method of manufacturing expanded polymeric foam according to claim19, wherein the carbon dioxide comprises less than about 4% by weight ofthe foamable mixture.
 23. A method of manufacturing expanded polymericfoam according to claim 19, wherein the at least one additive comprisesfrom about 0.5% by weight to about 1.5% by weight of the foamablemixture.
 24. A foam board manufactured by the method of claim
 19. 25. Amethod of manufacturing expanded polymeric foam according to claim 19,wherein the at least one additive provides a density of about 2.25 pcfor less in the expanded polymeric foam.
 26. A method of manufacturingexpanded polymeric foam according to claim 19, wherein the at least oneadditive provides an R-value of at least 4.2° F.·ft2·hr/BTU per inch inthe expanded polymeric foam.
 27. A method of manufacturing expandedpolymeric foam according to claim 19, wherein the polymer compositionand the at least one additive are precompounded prior to the injectionof the two or more blowing agents.